1. Field of the Invention
The present invention relates to a focus detecting device adapted for use in a camera or the like.
2. Related Background Art
There is already known, for use in a camera or the like, a focus detecting device of so-called divided-pupil re-imaging system, which is based on the following principle. At first a primary image, formed by a pair of light beams passing through mutually different two areas on the pupil of a phototaking optical system, is re-imaged by a pair of re-imaging optical elements to form a pair of secondary images, which are then converted into a pair of electrical image signals by an image sensor provided with a pair of light-receiving units. The relative positional relationship of said paired electrical image signals is detected by a correlation calculation, and the focus state of said phototaking optical system relative to an anticipated focal plane (for example a film plane) is detected from thus detected distance of the images.
In such focus detecting device, each of said paired light-receiving units is composed of a one-dimensional array of plural pixels, and, if such paired light-receiving units are projected by the corresponding re-imaging optical elements onto said anticipated focal plane, the obtained projected images substantially coincide mutually, and the focus detecting area is defined by the shape of such projected images. Consequently, the focus detecting area is usually elongated one-dimensionally, corresponding to the projected images of said light-receiving units.
Such a conventional focus detecting device are disclosed in, for example, Japanese Patent Application Laid-open Nos. 55-111928, 59-65814, 62-86318, 1-81924, 1-189619, 4-332015, 4-277712, 4-277713, 5-264887, 6-186473, 6-265774, 6-273665 or 6-308379 or U.S. Pat. Nos. 4,560,863 or 4,563,576 or Japanese Patent Application No. 5-140975 filed on Jun. 11, 1993.
In such conventional focus detecting device, it has been difficult to cover the wide range of luminance, of about 20 EV, of the object, variable from a high luminance state to a low luminance state, even by the adjustment of the amplifying gain of the sensor output and of the charge accumulating time of the pixels, since the light-receiving area of the pixels constituting the light-receiving units is fixed.
In order to avoid such drawback, it is conceivable to form, in the vicinity of said light-receiving units, another pair of light-receiving units of a larger pixel area and to detect the focus state, in a low luminance state, by the outputs of the latter light-receiving units. Since the focus detecting area should preferably remain the same as much as possible for the low luminance state and for the high luminance state, the pixel array constituting the light-receiving units of the smaller pixel area and that constituting the light-receiving units of the larger pixel area are arranged mutually parallel and mutually close, and a same pair of re-imaging elements are employed for projecting different portions of same object images on said two pairs of light-receiving units.
Also for increasing the area of each pixel while maintaining the accuracy of focus detection, it is desirable to increase the dimension of the pixels in a direction perpendicular to the direction of array of the pixels, while not varying the pitch of array of the pixels. As a result, if such light-receiving units of the larger pixel area is inversely projected onto the anticipated focal plane, the width of the projected image (in a direction perpendicular to the direction of one-dimensional array of the pixels) becomes larger than the width of the projected image, obtained when the light-receiving units of the smaller pixel area are inversely projected onto the anticipated focal plane.
The above-explained light-receiving units formed in two pairs can be adopted for avoiding the aforementioned drawback, but, as the projected images of the two-paired light-receiving units on the anticipated focal plane do not mutually overlap, the focus detection may be erroneously conducted for the background in case the object is small and cannot be properly captured by such light-receiving units.
There has not so far been provided a device in which the positional relationship between the phototaking frame or the focus frame, indicating the focus detecting area, and the projected images of two pairs of the light-receiving areas, in consideration of the difference in shape of the projected images of the two pairs of the light-receiving units onto the anticipated focal plane and of the general rule of the object (objects being often distributed in the vertical direction, along the direction of gravity) for preventing the aforementioned drawback.